Particle separation and sorting is a common practice in which particulate matter of a variety of shapes and sizes are categorized based on the physical attributes of the individual particles. Commonly, this is accomplished through the use of sieves. Similar to the common household kitchen tool, industrial sieves of will often be stacked on top of one another with the largest mesh size at the top, such that as particles fall down they are eventually caught in the increasingly fine stages of the mesh. This process, while quite standard in industry, suffers from a few drawbacks. First, it relies on multiple levels of sieves for each particular size range. As one might imagine, as particle sizes of interest approach diameters on the order of tens of micrometers, fabrication of these wire-mesh devices can become costly and difficult. Additionally, with wire meshes of this size, fragility becomes an issue and the devices are easily damaged. Second, the devices rely on a gravitational field to be effective. The presence of such a field may be a safe assumption in the vast majority of cases, but in the case of space exploration it is not. For a rover on a distant planet, moon, or even asteroid, gravity cannot be relied upon to aid in particle sorting for scientific analysis.
In another attempt to address the needs for particle sorting, particles of different sizes were accelerated through inkjet nozzles and, after a charging step, were able to efficiently sort particles based on their size. However, this method was inherently limited in terms of the particle size because of the size of the inkjet nozzle orifice. In another attempt, AC electric fields were used to charge and convey particles around a 2D surface. Using electrodes patterned on the planar surface, particles sitting on a thin insulating layer could be effectively conveyed, smoothed, or sorted by varying the applied electric fields.
Additionally, multiple attempts include methods based on dielectrophoresis were used. In these attempts, dielectric particles were sorted by the application of electric fields to flows containing particles in microfluidic channels. This sorting can be done continuously, but requires that the particles be suspended in a fluid flow. In a further attempt a solution based on pressure driven flow combined with electro-osmosis was presented. Using a multi channeled microfluidic device, particles or cells from a single channel were sorted into one of two channels electrically and, once separated, were carried away from the sorting area by pressure driven flow. Non-electric micro-fluidic processes have also been attempted that include a particle separation device that relies only on gravity. This allowed for the separation of particles as they flowed horizontally due to the fact that the more massive particles would descend further as the flow traveled over a specified distance due to the graviational field, allowing them to be isolated after sufficient physical separation had occurred for separating particles without the aid of an electrical field or gravity. The process began by suspending particles in a fluid and forcing the fluid to flow through a micro-fluidic main channel. Off of a main channel were perpendicular channels into which the flow of fluid was controlled. By regulating the flow rate into the side channels, the size of particles were controlled that would be drawn in to them as they moved through the main channel, thus allowing the particles to be sorted based on their size.
In yet another attempt, a magnetically driven micro-tool for sorting particles. A MEMS device was fabricated that was actuated on a rotational joint, allowing it to take on one of two positions. Each of the positions would divert the flow of particles and suspending liquid to a separate channel. By actively controlling the tool, particles suspended in a liquid could be sorted in a binary fashion.
Each of these solutions, however, requires a complex device or devices as well as input energy. Previously, these requirements were of little import as the majority of particle separation and sorting work was done in industrial or laboratory environments where these resources were available.
What is needed is a method of separating particles in the field, without large pieces of equipment for separation conducted in an extreme or remote environment where power is at a premium and gravity and human interaction cannot be relied upon, a sufficiently low-cost, low-complexity, method must be developed.